Dynamically load CodeMirror themes

Most users run with the default theme, which does not require any
additional CSS. The total set of themes costs 7.3 KiB of download
when added into the default CSS file.

Split the themes into their own files and only load the theme when
it is required by the browser.

Change-Id: I48f274347e1ca94895c0756fa17479661c78fd57
diff --git a/gerrit-gwtui/src/main/java/net/codemirror/CodeMirror.gwt.xml b/gerrit-gwtui/src/main/java/net/codemirror/CodeMirror.gwt.xml
index 24a0f57..add033f 100644
--- a/gerrit-gwtui/src/main/java/net/codemirror/CodeMirror.gwt.xml
+++ b/gerrit-gwtui/src/main/java/net/codemirror/CodeMirror.gwt.xml
@@ -20,4 +20,5 @@
   <source path='lib'/>
   <source path='keymap'/>
   <source path='mode'/>
+  <source path='theme'/>
 </module>